When police mounted a major drugs bust on a cannabis factory in Northern Ireland last year they uncovered hard and shocking evidence of a link between the drug barons and human trafficking.

Inside this makeshift 'factory' in Belfast a Chinese man was being held captive and forced to work as a 'gardener' night and day, tending to the cannabis plants.

The man had been trafficked into the province and was working to pay off his debt to his traffickers.

"That poor gentleman from China didn't even know he was in Belfast. These gardeners are located in these premises to look after the plants. They are given a little food and are forced to work to pay off their debt for their illegal immigration," Detective Chief Inspector Shaun McKee (right) of the PSNI's Organised Crime Branch said.

Crime gangs are running their cannabis factories or 'farms' from properties across Northern Ireland. The farm could be in a mid- terrace house in suburban south Belfast.

Inside these houses, which from the outside could look normal, floors have been ripped up, walls knocked down and tunnels dug. In most cases the electricity supply has been bypassed, causing a risk of fire and electrocution.

For several years Triad gangs and other gangsters from south-east Asia were heavily involved in the cultivation of cannabis here for sale across Ireland, the UK and Europe. At the height of the trade more than 100 factories were uncovered by police in one year. A crackdown by officers has seen the displacement of these gangs out of Northern Ireland and a significant reduction in the number of factories.

Cannabis remains the drug of choice here with crime bosses knowing that supply of class B drugs like cannabis can lead to a 14-year jail term, compared to a life sentence for supplying class A drugs like cocaine or heroin. "If the gangs can trade and make money from class B and class C drugs, from their point of view they would prefer to take the risk on class B and C than class A," DCI McKee explained.

However, there is still a large market for cocaine in Northern Ireland, which is making massive amounts of money for the gangs.

The average street purity of cocaine in Northern Ireland is as little as 7%, with the drug bulked out with a hazardous cocktail of chemicals.

"I have seen purity levels as low as 1% and up as high as 89%. The rest is made up of other cutting agents from the veterinary arena. The effect of the agents creates numbing of the nose: they snort it and think it is great and it is having no effect whatsoever, and they are paying £60 for a gram of coke for the pleasure."

He added: "You get some idiots going out to places like Colombia, Peru and Bolivia on coke holidays as that is where the pure coke is. The closer you go to the source country to buy, then the purer it is.

"The purity of coke found in someone's possession has a bearing on the sentence, because it shows where they sit on the food chain. You wouldn't be in possession of high-purity cocaine unless you are a drug trafficker or trusted associate."

The war on drugs is one of the few issues on which the police receive full support from all communities in Northern Ireland.

Last year after police officers came under attack during riots in Ardoyne, the next day they were receiving support from that same community during a drugs operation in the north Belfast neighbourhood.

"The very next day we did a bread-and-butter drugs operations in Ardoyne and we got many supportive phone calls coming in on the back of that. This was normal policing in that community. We even received a phone call from one individual who, in my past, would never phone police," said Mr McKee.

He added: "Every community supports us in this. Drugs destroy lives and communities. I believe we are definitely making a difference."